Methods to avoid frequency locking in a multi-channel neurostimulation system using pulse placement

ABSTRACT

A method and neurostimulation system for treating a patient are provided. A plurality of pulsed electrical waveforms are respectively delivered within a plurality of timing channels of the neurostimulation system, thereby treating the patient. Sets of stimulation pulses within the electrical waveforms that will potentially overlap temporally are predicted. Each of the potentially overlapping pulse sets is substituted with a replacement stimulation pulse, such that each replacement stimulation pulse is delivered within at least one of the respective timing channels, thereby preventing temporal overlap between the stimulation pulses of the respective electrical waveforms while preventing frequency locking between the timing channels.

FIELD OF THE INVENTION

The present invention relates to tissue stimulation systems, and moreparticularly, to a system and method for eliminating or reducingfrequency locking in multi-channel neurostimulation systems.

BACKGROUND OF THE INVENTION

Implantable neurostimulation systems have proven therapeutic in a widevariety of diseases and disorders. Pacemakers and Implantable CardiacDefibrillators (ICDs) have proven highly effective in the treatment of anumber of cardiac conditions (e.g., arrhythmias). Spinal CordStimulation (SCS) systems have long been accepted as a therapeuticmodality for the treatment of chronic pain syndromes, and theapplication of tissue stimulation has begun to expand to additionalapplications such as angina pectoralis and incontinence. Deep BrainStimulation (DBS) has also been applied therapeutically for well over adecade for the treatment of refractory chronic pain syndromes, and DBShas also recently been applied in additional areas such as movementdisorders and epilepsy. Further, in recent investigations, PeripheralNerve Stimulation (PNS) systems have demonstrated efficacy in thetreatment of chronic pain syndromes and incontinence, and a number ofadditional applications are currently under investigation. Furthermore,Functional Electrical Stimulation (FES) systems, such as the Freehandsystem by NeuroControl (Cleveland, Ohio), have been applied to restoresome functionality to paralyzed extremities in spinal cord injurypatients.

These implantable neurostimulation systems typically include one or moreelectrode carrying stimulation leads, which are implanted at the desiredstimulation site, and a neurostimulator (e.g., an implantable pulsegenerator (IPG)) implanted remotely from the stimulation site, butcoupled either directly to the stimulation lead(s) or indirectly to thestimulation lead(s) via a lead extension. The neurostimulation systemmay further comprise an external control device to remotely instruct theneurostimulator to generate electrical stimulation pulses in accordancewith selected stimulation parameters.

Electrical stimulation energy may be delivered from the neurostimulatorto the electrodes in the form of an pulsed electrical waveform. Thus,stimulation energy may be controllably delivered to the electrodes tostimulate neural tissue. The combination of electrodes used to deliverelectrical pulses to the targeted tissue constitutes an electrodecombination, with the electrodes capable of being selectively programmedto act as anodes (positive), cathodes (negative), or left off (zero). Inother words, an electrode combination represents the polarity beingpositive, negative, or zero. Other parameters that may be controlled orvaried include the amplitude, duration, and rate of the electricalpulses provided through the electrode array. Each electrode combination,along with the electrical pulse parameters, can be referred to as a“stimulation parameter set.”

With some neurostimulation systems, and in particular, those withindependently controlled current or voltage sources, the distribution ofthe current to the electrodes (including the case of theneurostimulator, which may act as an electrode) may be varied such thatthe current is supplied via numerous different electrode configurations.In different configurations, the electrodes may provide current orvoltage in different relative percentages of positive and negativecurrent or voltage to create different electrical current distributions(i.e., fractionalized electrode configurations).

As briefly discussed above, an external control device can be used toinstruct the neurostimulator to generate electrical stimulation pulsesin accordance with the selected stimulation parameters. Typically, thestimulation parameters programmed into the neurostimulator can beadjusted by manipulating controls on the external control device tomodify the electrical stimulation provided by the neurostimulator systemto the patient. However, the number of electrodes available combinedwith the ability to generate a variety of complex stimulation pulses,presents a vast selection of stimulation parameter sets to the clinicianor patient.

To facilitate such selection, the clinician generally programs theneurostimulator through a computerized programming system. Thisprogramming system can be a self-contained hardware/software system, orcan be defined predominantly by software running on a standard personalcomputer (PC). The PC or custom hardware may actively control thecharacteristics of the electrical stimulation generated by theneurostimulator to allow the optimum stimulation parameters to bedetermined based on patient feedback or other means and to subsequentlyprogram the neurostimulator with the optimum stimulation parameter setor sets, which will typically be those that stimulate all of the targettissue in order to provide the therapeutic benefit, yet minimizes thevolume of non-target tissue that is stimulated. The computerizedprogramming system may be operated by a clinician attending the patientin several scenarios.

Often, multiple timing channels are used when applying electricalstimulation to target different tissue regions in a patient. Forexample, in the context of SCS, the patient may simultaneouslyexperience pain in different regions (such as the lower back, left arm,and right leg) that would require the electrical stimulation ofdifferent spinal cord tissue regions. In the context of DBS, a multitudeof brain structures may need to be electrically stimulated in order tosimultaneously treat ailments associated with these brain structures.Each timing channel identifies the combination of electrodes used todeliver electrical pulses to the targeted tissue, as well as thecharacteristics of the current (pulse amplitude, pulse duration, pulsefrequency, etc.) flowing through the electrodes.

The use of multiple timing channels can often lead to problems with theelectrical stimulation systems due to the potential of an overlap inpulses between two or more timing channels. Overlapping of pulses usinga common electrode can make neurostimulation systems ineffective or evenharmful. Current neurostimulation systems employing multiple timingchannels use a method known as the “token” method to prevent overlap ofpulses. This method allows an electrical pulse to be transmitted in thetiming channel with the “token,” while the other timing channels waittheir turn. Then, the “token” is passed to the next timing channel.However, if the frequencies of the channels overlap, such that they needthe “token” at the same time, transmission of an electrical pulse withinthe second channel must wait until the end of the transmission of theelectrical pulse in the first timing channel. One possible result isthat the frequency of the electrical pulses transmitted in the secondtiming channel gets “locked” to (i.e. matches) the frequency of theelectrical pulses transmitted in the first timing channel;alternatively, one can get galloping or clumping of electrical pulses.Therefore, when the occurrence of stimulation pulses is pushed out intime, stimulation therapy becomes ineffective or even harmful for tissueregions, such as brain structures to be stimulated in DBS applications,that require stimulation at specific, regular frequencies.

The “token” method may best be understood with reference to FIG. 1. Asthere shown, a first pulsed electrical waveform 5 a having a firstfrequency is transmitted within timing channel A, and a second pulsedelectrical waveform 5 b having a second frequency is desired to betransmitted within timing channel B. Because timing channel A has the“token,” the pulses of the second pulsed electrical waveform 5 b thatare to be transmitted in timing channel B must be “bumped” each timethey overlap with the pulses of the first pulsed electrical waveform 5a. As can be seen in the bumped pulsed electrical waveform 5 c, when apulse is bumped (shown by the horizontal arrows), the next pulse relieson the new (bumped) pulse for timing. Thus, the next pulse is “doublebumped”: once when the previous pulse is bumped and a second time whenit overlaps a pulse of the pulsed electrical waveform 5 a transmitted inthe timing channel A. As a result, the frequency of the pulses in thesecond pulsed electrical waveform 5 b is forced (i.e., locked) into thefrequency for the first pulsed electrical waveform 5 a, resulting in apulsed electrical waveform 5 d that has a frequency twice as small asthe desired frequency.

There, thus, remains a need to provide an improved method for preventingor minimizing frequency locking within multi-channel neurostimulationsystems.

SUMMARY OF THE INVENTION

In accordance with a first aspect of the present inventions, a methodfor treating a patient using a multi-channel neurostimulation system isprovided. The method comprises delivering a plurality of pulsedelectrical waveforms respectively within a plurality of timing channelsof the neurostimulation system, thereby treating the patient. In onemethod, the pulsed electrical waveforms are delivered via a commonelectrode and have different pulse frequencies. The pulsed electricalwaveforms may, e.g., be defined in response to a user input. The methodfurther comprises predicting sets of stimulation pulses within theelectrical waveforms that will potentially overlap temporally. In onemethod, the stimulation pulses of each of the potentially overlappingpulse sets have the same polarity.

The method further comprises substituting each of the potentiallyoverlapping pulse sets with a replacement stimulation pulse, such thateach replacement stimulation pulse is delivered within at least one ofthe respective timing channels, thereby preventing temporal overlapbetween the stimulation pulses of the respective electrical waveformswhile preventing frequency locking between the timing channels. Anoptional method further comprises predicting a charge recovery pulse anda stimulation pulse within the electrical waveforms that willpotentially overlap temporally, and dropping or temporally shifting atleast a portion of the charge recovery pulse, thereby preventingtemporal overlap between the charge recovery pulse and the stimulationpulse of the respective electrical waveforms. In one embodiment, eachreplacement stimulation pulse is delivered within all of the respectivetiming channels.

In one method, substituting each potentially overlapping pulse set withthe replacement stimulation pulse comprises selecting one of thestimulation pulses of each potentially overlapping pulse set as thereplacement stimulation pulse. As one example, the method may furthercomprise determining the relative amplitude or pulse duration of thestimulation pulses within each potentially overlapping pulse set, inwhich case, the one stimulation pulse of each potentially overlappingpulse set can be selected as the respective replacement stimulationpulse based on the respective determined relative amplitude or pulseduration. As another example, the selection of the one stimulation pulseof each potentially overlapping pulse set alternates between therespective timing channels. As still another example, the method mayfurther comprise assigning one of the timing channels as a high prioritytiming channel, in which case, the one stimulation pulse of eachpotentially overlapping pulse set associated with the high prioritytiming channel can be selected as the replacement stimulation pulse. Thetiming channels may be respectively associated with different tissueregions, and the timing channel associated with the tissue region thatwould be more adversely affected by dropping a stimulation pulse withinthe timing channel can be assigned as the high priority timing channel.As yet another example, one stimulation pulse of at least one of thepotentially overlapping pulse sets is a cathodic pulse, and anotherstimulation pulse of the at least one potentially overlapping pulse setis an anodic pulse, and the cathodic pulse is selected as thereplacement stimulation pulse. In still another example, one stimulationpulse of at least one of the potentially overlapping pulse sets is ananodic pulse, and another stimulation pulse of the at least onepotentially overlapping pulse set is a cathodic pulse, and the anodicpulse is selected as the replacement stimulation pulse

In another method, rather than selecting one of the stimulation pulsesof each potentially overlapping pulse set as the replacement stimulationpulse, each replacement stimulation pulse is defined as a function ofthe stimulation pulses within the respective potentially overlappingpulse set. In one example, the function comprises an average of theamplitudes and/or durations of the stimulation pulses within therespective potentially overlapping pulse set. In another example, thefunction comprises a summation of the amplitudes of the stimulationpulses within the respective potentially overlapping pulse set.

In accordance with a second aspect of the present inventions, amulti-channel neurostimulation system is provided. The neurostimulationsystem comprises a plurality of electrical terminals configured forbeing respectively coupled to a plurality of electrodes, and analogoutput circuitry configured for delivering a plurality of pulsedelectrical waveforms respectively within a plurality of timing channelsto the electrical terminals. In one embodiment, the stimulation pulsesof each of the potentially overlapping pulse sets have the samepolarity. In another embodiment, the analog output circuitry isconfigured for delivering the pulsed electrical waveforms via a commonelectrode. In another embodiment, the pulsed electrical waveforms havedifferent pulse frequencies.

The neurostimulation system further comprises control circuitryconfigured for predicting sets of stimulation pulses within the pulsedelectrical waveforms that will potentially overlap temporally, andsubstituting each of the potentially overlapping pulse sets with areplacement stimulation pulse, such that each replacement stimulationpulse is delivered within at least one of the respective timingchannels, thereby preventing temporal overlap between the stimulationpulses of the respective pulsed electrical waveforms. In an optionalembodiment, the control circuitry is further configured for predicting acharge recovery pulse and a stimulation pulse within the electricalwaveforms that will potentially overlap temporally, and dropping ortemporally shifting at least a portion of the charge recovery pulse,thereby preventing temporal overlap between the charge recovery pulseand the stimulation pulse of the respective electrical waveforms. In oneembodiment, the analog output circuitry is configured for deliveringeach replacement stimulation pulse within all of the respective timingchannels. In another embodiment, the control circuitry is furtherconfigured for defining the pulsed electrical waveforms in response to auser input.

In one embodiment, the control circuitry is configured for substitutingeach potentially overlapping pulse set with the replacement stimulationpulse comprises by selecting one of the stimulation pulses of eachpotentially overlapping pulse set as the replacement stimulation pulse.As one example, the control circuitry is further configured fordetermining the relative amplitude or pulse duration of the stimulationpulses within each potentially overlapping pulse set, wherein thecontrol circuitry is configured for selecting the one stimulation pulseof each potentially overlapping pulse set as the respective replacementstimulation pulse based on the respective determined relative amplitudeor pulse duration. As another example, the one stimulation pulse of eachpotentially overlapping pulse set alternates between the respectivetiming channels. As still another example, the control circuitry isfurther configured for assigning one of the timing channels as a highpriority timing channel, and selecting the one stimulation pulse of eachpotentially overlapping pulse set associated with the high prioritytiming channel as the replacement stimulation pulse. The timing channelsmay be respectively associated with different tissue regions, and thecontrol circuitry can be configured for assigning the timing channelassociated with the tissue region that would be more adversely affectedby dropping a stimulation pulse within the timing channel as the highpriority timing channel. As yet another example, one of the stimulationpulses of at least one of the potentially overlapping pulse sets is acathodic pulse, and another pulse of the at least one potentiallyoverlapping pulse set is an anodic pulse, in which case, the controlcircuitry can be configured for selecting the cathodic pulse as thereplacement stimulation pulse.

In another embodiment, rather than selecting one of the stimulationpulses of each potentially overlapping pulse set as the replacementstimulation pulse, the control circuitry is further configured fordefining each of the replacement stimulation pulses as a function of thestimulation pulses within the respective potentially overlapping pulseset. In one example, function comprises an average of the amplitudesand/or durations of the stimulation pulses within the respectivepotentially overlapping pulse set. In another example, the functioncomprises a summation of the amplitudes of the stimulation pulses withinthe respective potentially overlapping pulse set.

Other and further aspects and features of the invention will be evidentfrom reading the following detailed description of the preferredembodiments, which are intended to illustrate, not limit, the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings illustrate the design and utility of preferred embodimentsof the present invention, in which similar elements are referred to bycommon reference numerals. In order to better appreciate how theabove-recited and other advantages and objects of the present inventionsare obtained, a more particular description of the present inventionsbriefly described above will be rendered by reference to specificembodiments thereof, which are illustrated in the accompanying drawings.Understanding that these drawings depict only typical embodiments of theinvention and are not therefore to be considered limiting of its scope,the invention will be described and explained with additionalspecificity and detail through the use of the accompanying drawings inwhich:

FIG. 1 is timing diagram illustrating a prior art technique forpreventing the overlap between pulses of pulsed electrical waveformsprogrammed in multiple timing channels;

FIG. 2 is a plan view of an embodiment of a deep brain stimulation (DBS)system arranged in accordance with the present inventions;

FIG. 3 is a profile view of an implantable pulse generator (IPG) andpercutaneous leads used in the DBS system of FIG. 2;

FIG. 4 is a plot of monophasic cathodic electrical stimulation energy;

FIG. 5 a is a plot of biphasic electrical stimulation energy having acathodic stimulation pulse and an active charge recovery pulse;

FIG. 5 b is a plot of biphasic electrical stimulation energy having acathodic stimulation pulse and a passive charge recovery pulse;

FIG. 6 is a plan view of the DBS system of FIG. 2 in use with a patient;

FIG. 7 is a block diagram of the internal components of the IPG of FIG.3;

FIG. 8 is a timing diagram of two pulsed electrical waveforms deliveredwithin two respective timing channels of the IPG of FIG. 3, whereinpulses of the respective electrical waveforms temporally overlap witheach other;

FIG. 9 is a timing diagram of two pulsed electrical waveforms deliveredwithin two respective timing channels of the IPG of FIG. 3, wherein afirst technique is used to prevent temporal overlap between the pulsesof the respective electrical waveforms;

FIG. 10 is a timing diagram of two pulsed electrical waveforms deliveredwithin two respective timing channels of the IPG of FIG. 3, wherein asecond technique is used to prevent temporal overlap between the pulsesof the respective electrical waveforms;

FIG. 11 is a timing diagram of two pulsed electrical waveforms deliveredwithin two respective timing channels of the IPG of FIG. 3, wherein athird technique is used to prevent temporal overlap between the pulsesof the respective electrical waveforms;

FIG. 12 is a timing diagram of two pulsed electrical waveforms deliveredwithin two respective timing channels of the IPG of FIG. 3, wherein afourth technique is used to prevent temporal overlap between the pulsesof the respective electrical waveforms;

FIG. 13 is a timing diagram of two pulsed electrical waveforms deliveredwithin two respective timing channels of the IPG of FIG. 3, wherein afifth technique is used to prevent temporal overlap between the pulsesof the respective electrical waveforms;

FIG. 14 is a timing diagram of two pulsed electrical waveforms deliveredwithin two respective timing channels of the IPG of FIG. 3, wherein asixth technique is used to prevent temporal overlap between the pulsesof the respective electrical waveforms;

FIG. 15 is a timing diagram of two pulsed electrical waveforms deliveredwithin two respective timing channels of the IPG of FIG. 3, wherein aseventh technique is used to prevent temporal overlap between the pulsesof the respective electrical waveforms;

FIG. 16 is a timing diagram of two pulsed electrical waveforms deliveredwithin two respective timing channels of the IPG of FIG. 3, whereinpulses of the respective electrical waveforms temporally overlap witheach other;

FIG. 17 is a timing diagram of two pulsed electrical waveforms deliveredwithin two respective timing channels of the IPG of FIG. 3, wherein aneighth technique is used to prevent temporal overlap between the pulsesof the respective electrical waveforms;

FIG. 18 is a timing diagram of two pulsed electrical waveforms deliveredwithin two respective timing channels of the IPG of FIG. 3, wherein aninth technique is used to prevent temporal overlap between the pulsesof the respective electrical waveforms;

FIG. 19 is a timing diagram of two pulsed electrical waveforms deliveredwithin two respective timing channels of the IPG of FIG. 3, wherein atenth technique is used to prevent temporal overlap between the pulsesof the respective electrical waveforms;

FIG. 20 is a timing diagram of two pulsed electrical waveforms deliveredwithin two respective timing channels of the IPG of FIG. 3, wherein aneleventh technique is used to prevent temporal overlap between thepulses of the respective electrical waveforms;

FIG. 21 is a timing diagram of two pulsed electrical waveforms deliveredwithin two respective timing channels of the IPG of FIG. 3, wherein atwelfth technique is used to prevent temporal overlap between the pulsesof the respective electrical waveforms; and

FIG. 22 is a timing diagram of two pulsed electrical waveforms deliveredwithin two respective timing channels of the IPG of FIG. 3, wherein athirteenth technique is used to prevent temporal overlap between thepulses of the respective electrical waveforms.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The description that follows relates to a deep brain stimulation (DBS)system. However, it is to be understood that the while the inventionlends itself well to applications in DBS, the invention, in its broadestaspects, may not be so limited. Rather, the invention may be used withany type of implantable electrical circuitry used to stimulate tissue.For example, the present invention may be used as part of a pacemaker, adefibrillator, a cochlear stimulator, a retinal stimulator, a stimulatorconfigured to produce coordinated limb movement, a cortical stimulator,a deep brain stimulator, peripheral nerve stimulator, microstimulator,or in any other neural stimulator configured to treat urinaryincontinence, sleep apnea, shoulder sublaxation, headache, etc.

Turning first to FIG. 2, an exemplary DBS neurostimulation system 10generally includes one or more (in this case, two) implantablestimulation leads 12, an implantable pulse generator (IPG) 14, anexternal remote controller RC 16, a clinician's programmer (CP) 18, anExternal Trial Stimulator (ETS) 20, and an external charger 22.

The IPG 14 is physically connected via one or more percutaneous leadextensions 24 to the stimulation leads 12, which carry a plurality ofelectrodes 26 arranged in an array. In the illustrated embodiment, thestimulation leads 12 are percutaneous leads, and to this end, theelectrodes 26 may be arranged in-line along the stimulation leads 12. Inalternative embodiments, the electrodes 26 may be arranged in atwo-dimensional pattern on a single paddle lead. As will be described infurther detail below, the IPG 14 includes pulse generation circuitrythat delivers electrical stimulation energy in the form of a pulsedelectrical waveform (i.e., a temporal series of electrical pulses) tothe electrode array 26 in accordance with a set of stimulationparameters.

The ETS 20 may also be physically connected via the percutaneous leadextensions 28 and external cable 30 to the stimulation leads 12. The ETS20, which has similar pulse generation circuitry as the IPG 14, alsodelivers electrical stimulation energy in the form of a pulse electricalwaveform to the electrode array 26 accordance with a set of stimulationparameters. The major difference between the ETS 20 and the IPG 14 isthat the ETS 20 is a non-implantable device that is used on a trialbasis after the stimulation leads 12 have been implanted and prior toimplantation of the IPG 14, to test the responsiveness of thestimulation that is to be provided.

The RC 16 may be used to telemetrically control the ETS 20 via abi-directional RF communications link 32. Once the IPG 14 andstimulation leads 12 are implanted, the RC 16 may be used totelemetrically control the IPG 14 via a bi-directional RF communicationslink 34. Such control allows the IPG 14 to be turned on or off and to beprogrammed with different stimulation parameter sets. The IPG 14 mayalso be operated to modify the programmed stimulation parameters toactively control the characteristics of the electrical stimulationenergy output by the IPG 14. As will be described in further detailbelow, the CP 18 provides clinician detailed stimulation parameters forprogramming the IPG 14 and ETS 20 in the operating room and in follow-upsessions.

The CP 18 may perform this function by indirectly communicating with theIPG 14 or ETS 20, through the RC 16, via an IR communications link 36.Alternatively, the CP 18 may directly communicate with the IPG 14 or ETS20 via an RF communications link (not shown). The clinician detailedstimulation parameters provided by the CP 18 are also used to programthe RC 16, so that the stimulation parameters can be subsequentlymodified by operation of the RC 16 in a stand-alone mode (i.e., withoutthe assistance of the CP 18).

The external charger 22 is a portable device used to transcutaneouslycharge the IPG 14 via an inductive link 38. For purposes of brevity, thedetails of the external charger 22 will not be described herein. Oncethe IPG 14 has been programmed, and its power source has been charged bythe external charger 22 or otherwise replenished, the IPG 14 mayfunction as programmed without the RC 16 or CP 18 being present.

For purposes of brevity, the details of the RC 16, CP 18, ETS 20, andexternal charger 22 will not be described herein. Details of exemplaryembodiments of these devices are disclosed in U.S. Pat. No. 6,895,280,which is expressly incorporated herein by reference.

Referring now to FIG. 3, the features of the stimulation leads 12 andthe IPG 14 will be briefly described. One of the stimulation leads 12(1)has eight electrodes 26 (labeled E1-E8), and the other stimulation lead12(2) has eight electrodes 26 (labeled E9-E16). The actual number andshape of leads and electrodes will, of course, vary according to theintended application. The IPG 14 comprises an outer case 40 for housingthe electronic and other components (described in further detail below),and a connector 42 to which the proximal ends of the stimulation leads12 mates in a manner that electrically couples the electrodes 26 to theelectronics within the outer case 40. The outer case 40 is composed ofan electrically conductive, biocompatible material, such as titanium,and forms a hermetically sealed compartment wherein the internalelectronics are protected from the body tissue and fluids In some cases,the outer case 40 may serve as an electrode.

As will be described in further detail below, the IPG 14 includes abattery and pulse generation circuitry that delivers the electricalstimulation energy in the form of a pulsed electrical waveform to theelectrode array 26 in accordance with a set of stimulation parametersprogrammed into the IPG 14. Such stimulation parameters may compriseelectrode combinations, which define the electrodes that are activatedas anodes (positive), cathodes (negative), and turned off (zero),percentage of stimulation energy assigned to each electrode(fractionalized electrode configurations), and electrical pulseparameters, which define the pulse amplitude (measured in milliamps orvolts depending on whether the IPG 14 supplies constant current orconstant voltage to the electrode array 26), pulse duration (measured inmicroseconds), pulse rate (measured in pulses per second), and burstrate (measured as the stimulation on duration X and stimulation offduration Y).

Electrical stimulation will occur between two (or more) activatedelectrodes, one of which may be the IPG case. Simulation energy may betransmitted to the tissue in a monopolar or multipolar (e.g., bipolar,tripolar, etc.) fashion. Monopolar stimulation occurs when a selectedone of the lead electrodes 26 is activated along with the case of theIPG 14, so that stimulation energy is transmitted between the selectedelectrode 26 and case. Bipolar stimulation occurs when two of the leadelectrodes 26 are activated as anode and cathode, so that stimulationenergy is transmitted between the selected electrodes 26. For example,electrode E3 on the first lead 12(1) may be activated as an anode at thesame time that electrode E11 on the second lead 12(1) is activated as acathode. Tripolar stimulation occurs when three of the lead electrodes26 are activated, two as anodes and the remaining one as a cathode, ortwo as cathodes and the remaining one as an anode. For example,electrodes E4 and E5 on the first lead 12 may be activated as anodes atthe same time that electrode E12 on the second lead 12 is activated as acathode

The stimulation energy may be delivered between a specified group ofelectrodes as monophasic electrical energy or multiphasic electricalenergy. As illustrated in FIG. 4, monophasic electrical energy includesa series of pulses that are either all negative (cathodic), oralternatively all positive (anodic). Multiphasic electrical energyincludes a series of pulses that alternate between positive andnegative.

For example, as illustrated in FIGS. 5 a and 5 b, multiphasic electricalenergy may include a series of biphasic pulses, with each biphasic pulseincluding a cathodic (negative) stimulation pulse (during a first phase)and an anodic (positive) charge recovery pulse (during a second phase)that is generated after the stimulation pulse to prevent direct currentcharge transfer through the tissue, thereby avoiding electrodedegradation and cell trauma. That is, charge is conveyed through theelectrode-tissue interface via current at an electrode during astimulation period (the length of the stimulation pulse), and thenpulled back off the electrode-tissue interface via an oppositelypolarized current at the same electrode during a recharge period (thelength of the charge recovery pulse).

The second phase may have an active charge recovery pulse (FIG. 5 a),wherein electrical current is actively conveyed through the electrodevia current or voltage sources, and a passive charge recovery pulse, orthe second phase may have a passive charge recovery pulse (FIG. 5 b),wherein electrical current is passively conveyed through the electrodevia redistribution of the charge flowing from coupling capacitancespresent in the circuit. Using active recharge, as opposed to passiverecharge, allows faster recharge, while avoiding the charge imbalancethat could otherwise occur. Another electrical pulse parameter in theform of an interphase can define the time period between the pulses ofthe biphasic pulse (measured in microseconds).

As shown in FIG. 6, the stimulation leads 12 are introduced through aburr hole 46 formed in the cranium 48 of a patient 44, and introducedinto the parenchyma of the brain 49 of the patient 44 in a conventionalmanner, such that the electrodes 26 are adjacent a target tissue regionwhose electrical activity is the source of the dysfunction (e.g., theventrolateral thalamus, internal segment of globus pallidus, substantianigra pars reticulate, subthalamic nucleus, or external segment ofglobus pallidus). Thus, stimulation energy can be conveyed from theelectrodes 26 to the target tissue region to change the status of thedysfunction. Due to the lack of space near the location where thestimulation leads 12 exit the burr hole 46, the IPG 14 is generallyimplanted in a surgically-made pocket either in the abdomen or above thebuttocks. The IPG 14 may, of course, also be implanted in otherlocations of the patient's body. The lead extension(s) 24 facilitateslocating the IPG 14 away from the exit point of the electrode leads 12.

Turning next to FIG. 7, the main internal components of the IPG 14 willnow be described. The IPG 14 includes stimulation output circuitry 50configured for generating electrical stimulation energy in accordancewith a defined pulsed waveform having a specified pulse amplitude, pulserate, pulse width, pulse shape, and burst rate under control of controllogic 52 over data bus 54. Control of the pulse rate and pulse width ofthe electrical waveform is facilitated by timer logic circuitry 56,which may have a suitable resolution, e.g., 10 μs. The stimulationenergy generated by the stimulation output circuitry 50 is output viacapacitors C1-C16 to electrical terminals 55 corresponding to theelectrodes 26. The analog output circuitry 50 may either compriseindependently controlled current sources for providing stimulationpulses of a specified and known amperage to or from the electrodes 26,or independently controlled voltage sources for providing stimulationpulses of a specified and known voltage at the electrodes 26.

Any of the N electrodes may be assigned to up to k possible groups or“channels.” In one embodiment, k may equal four. The channel identifieswhich electrodes are selected to synchronously source or sink current tocreate an electric field in the tissue to be stimulated. Amplitudes andpolarities of electrodes on a channel may vary, e.g., as controlled bythe RC 16. External programming software in the CP 18 is typically usedto set stimulation parameters including electrode polarity, amplitude,pulse rate and pulse duration for the electrodes of a given channel,among other possible programmable features.

The N programmable electrodes can be programmed to have a positive(sourcing current), negative (sinking current), or off (no current)polarity in any of the k channels. Moreover, each of the N electrodescan operate in a multipolar (e.g., bipolar) mode, e.g., where two ormore electrode contacts are grouped to source/sink current at the sametime. Alternatively, each of the N electrodes can operate in a monopolarmode where, e.g., the electrode contacts associated with a channel areconfigured as cathodes (negative), and the case electrode (i.e., the IPGcase) is configured as an anode (positive).

Further, the amplitude of the current pulse being sourced or sunk to orfrom a given electrode may be programmed to one of several discretecurrent levels, e.g., between 0 to 10 mA in steps of 0.1 mA. Also, thepulse duration of the current pulses is preferably adjustable inconvenient increments, e.g., from 0 to 1 milliseconds (ms) in incrementsof 10 microseconds (μs). Similarly, the pulse rate is preferablyadjustable within acceptable limits, e.g., from 0 to 1000 pulses persecond (pps). Other programmable features can include slow start/endramping, burst stimulation cycling (on for X time, off for Y time),interphase, and open or closed loop sensing modes.

The operation of this analog output circuitry 50, including alternativeembodiments of suitable output circuitry for performing the samefunction of generating stimulation pulses of a prescribed amplitude andduration, is described more fully in U.S. Pat. Nos. 6,516,227 and6,993,384, which are expressly incorporated herein by reference.

The IPG 14 further comprises monitoring circuitry 58 for monitoring thestatus of various nodes or other points 60 throughout the IPG 14, e.g.,power supply voltages, temperature, battery voltage, and the like. TheIPG 14 further comprises processing circuitry in the form of amicrocontroller (μC) 62 that controls the control logic over data bus64, and obtains status data from the monitoring circuitry 58 via databus 66. The IPG 14 additionally controls the timer logic 56. The IPG 14further comprises memory 68 and oscillator and clock circuitry 70coupled to the microcontroller 62. The microcontroller 62, incombination with the memory 68 and oscillator and clock circuit 70, thuscomprise a microprocessor system that carries out a program function inaccordance with a suitable program stored in the memory 68.Alternatively, for some applications, the function provided by themicroprocessor system may be carried out by a suitable state machine.

Thus, the microcontroller 62 generates the necessary control and statussignals, which allow the microcontroller 62 to control the operation ofthe IPG 14 in accordance with a selected operating program andstimulation parameters. In controlling the operation of the IPG 14, themicrocontroller 62 is able to individually generate a train of stimuluspulses at the electrodes 26 using the analog output circuitry 60, incombination with the control logic 52 and timer logic 56, therebyallowing each electrode 26 to be paired or grouped with other electrodes26, including the monopolar case electrode. In accordance withstimulation parameters stored within the memory 68, the microcontroller62 may control the polarity, amplitude, rate, pulse duration and channelthrough which the current stimulus pulses are provided. Themicrocontroller 62 also facilitates the storage of electrical parameterdata (or other parameter data) measured by the monitoring circuitry 58within memory 68, and also provides any computational capability neededto analyze the raw electrical parameter data obtained from themonitoring circuitry 58 and compute numerical values from such rawelectrical parameter data.

Significantly, as will be described in further detail below, themicrocontroller 62 uses a set of rules to prevent overlap of pulsesbetween multiple timing channels. Alternatively, functions such as themanagement of stimulation pulses and timing information may be performedin a digital state machine, with the microcontroller 62 having asupervisory role to manage information flow, e.g., sending stimulationparameters to the analog circuitry and/or converting sampled analog datainto a digital form, and then post-processing the digital data forstorage or transmission to the RC 16.

The IPG 14 further comprises an alternating current (AC) receiving coil72 for receiving programming data (e.g., the operating program and/orstimulation parameters) from the RC 16 (shown in FIG. 2) in anappropriate modulated carrier signal, and charging and forward telemetrycircuitry 74 for demodulating the carrier signal it receives through theAC receiving coil 72 to recover the programming data, which programmingdata is then stored within the memory 68, or within other memoryelements (not shown) distributed throughout the IPG 14.

The IPG 14 further comprises back telemetry circuitry 76 and analternating current (AC) transmission coil 78 for sending informationaldata sensed through the monitoring circuitry 58 to the RC 16. The backtelemetry features of the IPG 14 also allow its status to be checked.For example, when the RC 16 initiates a programming session with the IPG14, the capacity of the battery is telemetered, so that the externalprogrammer can calculate the estimated time to recharge. Any changesmade to the current stimulus parameters are confirmed through backtelemetry, thereby assuring that such changes have been correctlyreceived and implemented within the implant system. Moreover, uponinterrogation by the RC 16, all programmable settings stored within theIPG 14 may be uploaded to the RC 16. Significantly, the back telemetryfeatures allow raw or processed electrical parameter data (or otherparameter data) previously stored in the memory 68 to be downloaded fromthe IPG 14 to the RC 16, which information can be used to track thephysical activity of the patient.

The IPG 14 further comprises a rechargeable power source 80 and powercircuits 82 for providing the operating power to the IPG 14. Therechargeable power source 80 may, e.g., comprise a lithium-ion orlithium-ion polymer battery. The rechargeable battery 80 provides anunregulated voltage to the power circuits 82. The power circuits 82, inturn, generate the various voltages 84, some of which are regulated andsome of which are not, as needed by the various circuits located withinthe IPG 14. The rechargeable power source 80 is recharged usingrectified AC power (or DC power converted from AC power through othermeans, e.g., efficient AC-to-DC converter circuits, also known as“inverter circuits”) received by the AC receiving coil 72. To rechargethe power source 80, an external charger (not shown), which generatesthe AC magnetic field, is placed against, or otherwise adjacent, to thepatient's skin over the implanted IPG 14. The AC magnetic field emittedby the external charger induces AC currents in the AC receiving coil 72.The charging and forward telemetry circuitry 74 rectifies the AC currentto produce DC current, which is used to charge the power source 80.While the AC receiving coil 72 is described as being used for bothwirelessly receiving communications (e.g., programming and control data)and charging energy from the external device, it should be appreciatedthat the AC receiving coil 72 can be arranged as a dedicated chargingcoil, while another coil, such as coil 78, can be used forbi-directional telemetry.

It should be noted that the diagram of FIG. 7 is functional only, and isnot intended to be limiting. Those of skill in the art, given thedescriptions presented herein, should be able to readily fashionnumerous types of IPG circuits, or equivalent circuits, that carry outthe functions indicated and described, which functions include not onlyproducing a stimulus current or voltage on selected groups ofelectrodes, but also the ability to measure electrical parameter data atan activated or non-activated electrode.

Additional details concerning the above-described and other IPGs may befound in U.S. Pat. No. 6,516,227, U.S. Patent Publication No.2003/0139781, and U.S. patent application Ser. No. 11/138,632, entitled“Low Power Loss Current Digital-to-Analog Converter Used in anImplantable Pulse Generator,” which are expressly incorporated herein byreference. It should be noted that rather than an IPG, the DBS system 10may alternatively utilize an implantable receiver-stimulator (not shown)connected to leads 12. In this case, the power source, e.g., a battery,for powering the implanted receiver, as well as control circuitry tocommand the receiver-stimulator, will be contained in an externalcontroller inductively coupled to the receiver-stimulator via anelectromagnetic link. Data/power signals are transcutaneously coupledfrom a cable-connected transmission coil placed over the implantedreceiver-stimulator. The implanted receiver-stimulator receives thesignal and generates the stimulation in accordance with the controlsignals.

As briefly discussed above, the IPG 14 may be programmed by the CP 18(or alternatively the RC 16) to operate over multiple timing channels.The IPG 14 may prevent overlap between the electrical pulses generatedin the respective timing channels, and to do so without frequencylocking occurring between the timing channels. While the techniquesdescribed herein for preventing overlapping of electrical pulses andfrequency locking between timing channels lend themselves well when theelectrode combinations assigned to the respective timing channels haveone or more common electrodes, these techniques may be useful even ifthe electrode combinations assigned to the respective timing channelsare completely different from each other. These techniques will now bedescribed.

Referring first to FIG. 8, two timing channels (Channel A and Channel B)of the IPG 14 may be programmed by the CP 18 (or alternatively, the RC16) with two pulsed electrical waveforms 100 a, 100 b, respectively,which when delivered by the analog output circuitry 50 of the IPG 14,will provide treatment to the patient in which the IPG 14 has beenimplanted. The electrode combinations assigned to the respective timingchannels will typically be those that result in the treatment of twodifferent regions. As briefly discussed above, each timing channelidentifies the electrodes that are selected to synchronously source orsink current to create an electrical field in the tissue to bestimulated, and that the amplitude and polarities of electrodes assignedto each timing channel may vary. Notably, more than one pulsedelectrical waveform can be delivered within any particular timingchannel, such as those exemplified in U.S. Pat. No. 6,895,280, which hasbeen previously incorporated herein by reference. For purposes ofbrevity and clarity, however, only one pulsed electrical waveform isshown for each timing channel. Furthermore, although the pulsedelectrical waveforms illustrated in FIG. 8 are monophasic in nature, thepulsed electrical waveforms delivered during a timing channel can bemultiphasic in nature, as described in further detail below.

As seen in FIG. 8, without modification, certain sets of respectivestimulation pulses of the electrical waveforms 100 a, 100 b willtemporally overlap each other (either partially or completely). However,the microcontroller 62 of the IPG 14 may predict the sets of stimulationpulses that will potentially overlap each other temporally prior totheir delivery within the respective timing channels, and replace eachof these potentially overlapping pulse sets with a stimulation pulse,such that each replacement stimulation pulse is delivered within atleast one of the respective timing channels (and thus, delivered to theboth electrode combinations assigned to the timing channels in the casewhere the electrode combinations are the same for both timing channels),thereby preventing temporal overlap between the stimulation pulses ofthe respective pulsed electrical waveforms 100 a, 100 b while preventingfrequency locking between the timing channels. If delivered in bothtiming channels, the replacement stimulation pulse will preferably besimultaneously delivered within the timing channels. If the potentiallyoverlapping stimulation pulses that are replaced are displaced from eachother in time, then the replacement stimulation pulse may be slightlydisplaced or offset in time from the potentially overlapping pulses thatthey replace.

In one embodiment, the IPG 14 may determine the relative amplitudeand/or duration (width) of the pulses within each potentiallyoverlapping pulse set, and select the single replacement stimulationpulse for each potentially overlapping pulse set based on the determinedrelative amplitude and/or pulse duration, with each replacementstimulation pulse being delivered within both timing channels. Forexample, as shown in FIG. 9, for each potentially overlapping pulse set,the IPG 14 selects the stimulation pulse having the largest amplitude asthe replacement stimulation pulse. In this manner, any differencebetween the amount current delivered to the electrode combinations by aset of non-overlapping stimulation pulses of the respective pulsedelectrical waveforms 100 a, 100 b and the amount of current delivered tothe electrode combinations by the replacement stimulation pulse (whichwill have to be distributed amongst two combinations of electrodes) isminimized. Alternatively, the pulse with the largest duration may beselected as the replacement stimulation pulse, as shown in FIG. 10.

In another embodiment, the IPG 14 may define each of the replacementstimulation pulses as a function of the stimulation pulses within therespective potentially overlapping pulse set that is replaced, with eachreplacement stimulation pulse being delivered within both timingchannels. For example, as shown in FIG. 11, for each potentiallyoverlapping pulse set, the IPG 14 averages the amplitudes and the pulsewidths of the respective stimulation pulses within the pulse set anduses this average as the amplitude and pulsewidth of the replacementstimulation pulse. Alternatively, the IPG 14 may average only theamplitudes or only the durations of the respective pulses within thepulse set and use this average as the respective amplitude or durationof the replacement stimulation pulse.

As another example shown in FIG. 12, for each potentially overlappingpulse set, the IPG 14 sums the amplitudes of the respective stimulationpulses within the pulse set and uses this sum as the amplitude of thereplacement stimulation pulse. By summing the pulses in each potentiallyoverlapping pulse set, a sufficient amount of electrical currentdelivered in each timing channel will be ensured, so that the respectivetissue regions of the patient will be adequately stimulated. The IPG 14may limit the amplitude of each replacement stimulation pulse (e.g., 20mA) to prevent over-stimulation of either tissue regions, and inparticular, the tissue region associated with the timing channel havingthe lower amplitude pulses.

In still another embodiment, the IPG 14 may alternately select thestimulation pulse of the respective pulsed electrical waveforms 100 a,100 b as the replacement stimulation pulse for each of the potentiallyoverlapping pulse sets. Each replacement stimulation pulse is deliveredwithin both timing channels. For example, as shown in FIG. 13, the IPG14 selects the stimulation pulse in the pulsed electrical waveform 100 ato replace the first potentially overlapping pulse set, then selects thestimulation pulse in the pulsed electrical waveform 100 b to replace thesecond potentially overlapping pulse set, then selects the stimulationpulse in the pulsed electrical waveform 100 a to replace the thirdpotentially overlapping pulse set, etc. As shown in FIG. 14, eachreplacement stimulation pulse can be delivered within only the timingchannel from which it was selected. Essentially, the stimulation pulseof the respective potentially overlapping pulse set that is not selectedis suppressed, such that no stimulation pulse is delivered in the timingchannel when the selected stimulation pulse is delivered in the othertiming channel.

In yet another embodiment, the IPG 14 assigns one of the timing channelsas a high priority timing channel, and selects the stimulation pulseassociated with the high priority timing channel as the replacementstimulation pulse for a series of potentially overlapping pulse sets. Ifthe timing channels are respectively associated with different tissueregions, the timing channel associated with the tissue region that wouldbe more adversely affected by dropping a stimulation pulse within thetiming channel can be assigned as the high priority timing channel(e.g., in response to a user input via the CP 18 or RC 16). Eachreplacement stimulation pulse is delivered within the high prioritytiming channel. Essentially, the stimulation pulse of the lower prioritytiming channel is suppressed. For example, as shown in FIG. 15, the IPG14 assigns Timing Channel B as the high-priority channel, and selectsthe stimulation pulse of the pulsed electrical waveform 100 b as thereplacement stimulation pulse for all of the potentially overlappingpulse sets. As shown in FIG. 15, each replacement stimulation pulse isdelivered only in Timing Channel B. Alternatively, each replacementstimulation pulse can be delivered in both Timing Channels A and B.

This embodiment may be especially useful when stimulating a keystructure in the brain that requires highly regular pulsed frequencies,with the timing channel associated with the key structure having a highpriority. Also, although this embodiment is discussed in the context ofDBS, in occipital nerve stimulation, lesser occipital nerve stimulationmay be a lower priority than greater occipital nerve stimulation. Inthis case, the timing channel associated with the greater occipitalnerve stimulation will be given high priority, such that the stimulationpulse within the potentially overlapping pulse associated with greateroccipital nerve stimulation will be selected as the replacementstimulation pulse.

Although the stimulation pulses in the potentially overlapping pulsesets have been described as being cathodic, it should be noted that theoverlapping pulse sets can be anodic, in which case, the same techniquescan be applied. If one pulse in a potentially overlapping pulse set isanodic and another pulse in the same potentially overlapping pulse setis cathodic, other techniques can be utilized to resolve this conflict.For example, as illustrated in FIG. 16, two timing channels (Channel Aand Channel B) of the IPG 14 may be programmed by the CP 18 (oralternatively, the RC 16) with two pulsed electrical waveforms 100 c,100 d, respectively, which when delivered by the IPG 14, will providetreatment to the patient in which the IPG 14 has been implanted.

As with the pulsed electrical waveforms 100 a, 100 b illustrated in FIG.8, without modification, certain sets of respective pulses of theelectrical waveforms 100 c, 100 d will temporally overlap each other(either partially or completely). Again, the IPG 14 may determine thesets of pulses that will potentially overlap each other temporally priorto their delivery within the respective timing channels, and replaceeach of these potentially overlapping pulse sets with a pulse, such thateach pulse is delivered within at least one of the respective timingchannels (and thus, delivered to the both electrode combinationsassigned to the timing channels), thereby preventing temporal overlapbetween the pulses of the respective pulsed electrical waveforms 100 c,100 d.

In this embodiment, however, the cathodic pulse in each potentiallyoverlapping pulse set of the electrical waveforms 100 c, 100 d isselected as the replacement stimulation pulse, as illustrated in FIG.17. Significantly, cathodic pulses are often the stimulating pulses, andare therefore, more important than anodic pulses, which are generallynot stimulating. As such, retaining the cathodic pulse as thereplacement stimulation pulse, while discarding or suppressing theanodic pulse, may not adversely affect therapy. In applications wherethe anodic pulses are used as the stimulating pulses, the anodic pulsemay be retained as the replacement stimulation pulse, while the cathodicpulse is discarded or suppressed.

Instead of replacing each of the potentially overlapping pulse sets witha pulse in the manner discussed above with respect to FIGS. 9-17, themicrocontroller 62 of the IPG 14 may temporally shift pulses in therespective pulsed electrical waveforms in a manner that prevents overlapof the determined pulse sets while preventing frequency locking betweenthe timing channels.

In one embodiment, the IPG 14 alternately shifts stimulation pulseswithin the potentially overlapping pulse sets. For example, as shown inFIG. 18, the IPG 14 temporally shifts the stimulation pulse of thepulsed electrical waveform 100 a for the first potentially overlappingpulse set, temporally shifts the stimulation pulse of the pulsedelectrical waveform 100 b for the second potentially overlapping pulseset, temporally shifts the stimulation pulse of the pulsed electricalwaveform 100 a for the third potentially overlapping pulse set, etc.Notably, the IPG 14 shifts each of the stimulation pulses in thedirection that would minimize the amount that the stimulation pulses areshifted from their original position. For example, in FIG. 18, thestimulation pulse in the pulsed electrical waveform 100 a is shiftedforward in time for the first potentially overlapping pulse set, thestimulation pulse in the pulsed electrical waveform 100 b is shiftedbackward in time for the second potentially overlapping pulse set, andthe stimulation pulse in the pulsed electrical waveform 100 a is shiftedforward time for the third potentially overlapping pulse set.

In another embodiment, the IPG 14 temporally shifts one of thestimulation pulses of each potentially overlapping pulse set forward,and temporally shifts the other of the stimulation pulses of the eachpotentially overlapping pulse set backward. Notably, the IPG 14 shiftseach of the stimulation pulses in the direction that would minimize theamount that the stimulation pulses are shifted from their originalposition. For example, as shown in FIG. 19, the stimulation pulse in theelectrical waveform 100 a is shifted backward in time, and thestimulation pulse in the electrical waveform 100 b is shifted forward intime for the first potentially overlapping pulse set, the stimulationpulse in the electrical waveform 100 a is shifted forward in time, andthe stimulation pulse in the electrical waveform 100 b is shiftedbackward in time for the second potentially overlapping pulse set, andthe stimulation pulse in the electrical waveform 100 a is shiftedforward in time, and the stimulation pulse in the electrical waveform100 b is shifted backward in time for the third potentially overlappingpulse set

In still another embodiment, the IPG 14 determines which stimulationpulse of each of the potentially overlapping pulse sets would need to beshifted the least to prevent overlapping of the stimulation pulseswithin the respective potentially overlapping pulse set, and temporallyshifts that stimulation pulse within the timing channel.

For example, as shown in FIG. 20, for the second and third overlappingpulse sets, the stimulation pulses in the electrical waveform 100 awould need to be shifted forward the least as compared to thestimulation pulses in the electrical waveform 100 b, and thus, thestimulation pulses in the electrical waveform 100 a are temporallyshifted forward within the Timing Channel A. With respect to the firstoverlapping pulse set, the respective stimulation pulses of theelectrical waveforms 100 a, 100 b would need to be shifted forward anequal amount to prevent the overlapping of the stimulation pulses. Inthis case, selection of the stimulation pulse that is to be temporallyshifted can be performed arbitrarily or based on other criteria.

In the example illustrated in FIG. 20, the pulses in the electricalwaveforms 100 a and 100 b are temporally shifted forward within therespective Timing Channels A and B. It should be appreciated that thepulses may be temporally shifted backward to avoid overlapping of thepulses. Selection of whether the pulses are to be shifted forward orbackward may be determined based on any one of a variety of criteria.For example, the user or the system 10 may select the direction (eitherforward or backward) in which the pulses are to be shifted, or thedirection in which the pulses are to be shifted may alternate or berandomly or pseudo-randomly selected. In these examples, the pulse thatneeds to be shifted the least in the selected direction would be shiftedto prevent overlap.

In yet another embodiment, the IPG 14 predicts sets of stimulationpulses within the pulsed electrical waveforms that will not temporallyoverlap prior to their delivery within the respective timing channels,and temporally shifts at least one stimulation pulse in each of thepotentially non-overlapping pulse sets. Thus, when the stimulationpulses are getting closer to the potentially overlapping pulse set(e.g., 4 pulses away), the IPG 14 may slightly shift one or bothstimulation pulses of the pulsed electrical waveforms before theyoverlap. The advantage of this technique is that the stimulation pulseswill only need to slightly be shifted in time from their originalposition, so that the frequency of the original pulsed electricalwaveform is closer to the frequency of the modified pulsed electricalwaveform.

For example, as shown in FIG. 21, two timing channels (Channel A andChannel B) of the IPG 14 may be programmed by the CP 18 (oralternatively, the RC 16) with two pulsed electrical waveforms 100 e,100 f, respectively, which when delivered by the IPG 14, will providetreatment to the patient in which the IPG 14 has been implanted. As withthe pulsed electrical waveforms 100 a, 100 b illustrated in FIG. 8,without modification, certain sets of respective stimulation pulses ofthe electrical waveforms 100 e, 100 f will temporally overlap each other(either partially or completely).

However, rather than shifting only one or both of the stimulation pulsesin the potentially overlapping pulse set, the stimulation pulses of thepotentially non-overlapping pulse sets previous to the potentiallyoverlapping pulse set are temporally shifted to prevent the stimulationpulses from bunching up. In this case, the stimulation pulses of thepotentially non-overlapping pulse sets are shifted slightly backward, sothat when the stimulation pulse of the subsequent potentiallyoverlapping pulse set is shifted backward to prevent overlap, thedeviation of the spacings between the resulting stimulation pulses andthe spacings between the original unshifted stimulation pulses will beslight. Essentially, the frequency of the resulting pulsed electricalwaveforms will vary only slightly from the original frequency of theoriginal pulsed electrical waveforms.

Although the stimulation pulses in the potentially non-overlapping pulsesets and potentially overlapping pulse sets are illustrated as beingshifted forward time (essentially, decreasing the frequency of theelectrical waveform), it should be appreciated that the stimulationpulses may be shifted backward in time (essentially, increasing thefrequency of the electrical waveform).

In yet another embodiment, the IPG 14 temporally shifts one or both ofthe stimulation pulses in the potentially overlapping pulse set by arandomized amount. For the purposes of this specification, a randomvalue includes a pseudo-random value (i.e., a process that appearsrandom, but is not, and exhibits statistical randomness while beinggenerated by an entirely deterministic causal process). The value of therandom amount can be computed using a conventional pseudo-randomgenerator. The IPG 14 may determine the randomized amount of time thatthe stimulation pulse or pulses are shifted by multiplying a nominalpulse shift (e.g., the time shift used to prevent pulse overlap) with arandomization variable. In order to prevent ineffective treatment, theIPG 14 may limit the difference between the randomized amount and normaltime shift (e.g., 16 msec). For example, as shown in FIG. 22, thestimulation pulses in the electrical waveform 100 a are shifted forwarda randomized amount of time. As shown by the dashed lines in each of thepotentially overlapping pulse sets, the difference between therandomized shift and the nominal shift needed to prevent overlap betweenthe respective pulses is different for each of the potentiallyoverlapping pulse sets, indicating that that the pulses shifts arerandomized.

It should be noted that the embodiments illustrated in FIGS. 18-22 donot shift the stimulation pulses of a particular electrical waveformthat are subsequently delivered after a stimulation pulse that has beenshifted within the same electrical waveform if the subsequentstimulation pulses do not temporally overlap the pulses of the otherelectrical waveform. That is, the stimulation pulses in each pulsedelectrical waveform that do not overlap the stimulation pulses in theother pulsed electrical waveform or waveforms remain in their originalposition regardless of any shifting of other stimulation pulses. In thismanner, the frequency ratio between the respective pulsed electricalwaveforms remains substantially the same. Alternatively, however,stimulation pulses of a particular electrical waveform, even though theywould not temporally overlap with any stimulation pulse of the otherelectrical waveform or waveforms, may be shifted in order to maintain auniform spacing between the pulses of the electrical waveform as much aspossible. For example, if a stimulation pulse of a particular electricalwaveform is shifted forward in time, the next stimulation pulse may beshifted forward in time the same amount in order to maintain the nominalspacing between the respective stimulation pulses. In this manner, thefrequency of each pulsed electrical waveform is maintained as uniformlyas possible.

In the previous embodiments, the pulsed electrical waveforms areillustrated and describes as being monophasic in nature. It should beappreciated that the pulsed electrical waveforms may be multiphasic(e.g., biphasic) in nature. In this case, a charge recovery pulse(either passive or active) will accompany each stimulation pulse, asillustrated in FIGS. 5 a and 5 b. In this case, the IPG 14 attempts toprevent the overlap between a charge recovery pulse delivered within onetiming channel and a stimulation pulse delivered within another timingchannel. For example, the IPG 14 may predict a charge recovery pulse anda stimulation pulse within the pulsed electrical waveforms that willpotentially overlap temporally, and dropping or temporally shifting atleast a portion of the charge recovery pulse, thereby preventingtemporal overlap between the charge recovery pulse and the stimulationpulse of the respective electrical waveforms. In another embodiment, theIPG 14 drops or temporally shifts any charge recovery pulse associatedwith a stimulation pulse that is averaged or summed within anotherstimulation pulse (e.g., shown in FIGS. 11 and 12).

If a charge recovery phase is dropped or temporally shifted, the IPG 14may employ an interlock algorithm to make sure that there is chargerecovery after a certain number of drops or delays of the chargerecovery pulse, a certain number of stimulation pulses, the nextstimulation pulse, a set amount of time, an amount of time determined bya certain number of stimulation pulses, and/or a specified amount ofcharge is injected into the tissue (or before a specified amount isinjected). If a charge recovery pulse is interrupted, a countdown timemay be used to manage the length of the interrupted charge recoverypulse and make sure the remainder of the charge recovery pulse iscompleted.

The limit on the time between passive charge recovery pulses may also bedetermined by the amplitude, pulse width, and frequency of the pulses.For example, pulsed electrical waveforms with a high amplitude and shortpulse width may require passive charge recovery pulses less often.Alternatively, coupling capacity may be measured at the end of eachstimulation pulse to determine how much charge is injected into thetissue, and trigger the recharge pulse at the appropriate time and withthe appropriate duration. Measurement of the coupling capacity may beaccomplished by measuring the output bias on the output capacitors.

If a passive charge recovery pulse delivered in one timing channeltemporally overlaps an active charge recovery pulse in another timingchannel, the first charge recovery pulse in time may be interrupted toprevent overlap with the second charge recovery pulse in time.

Although particular embodiments of the present inventions have beenshown and described, it will be understood that it is not intended tolimit the present inventions to the preferred embodiments, and it willbe obvious to those skilled in the art that various changes andmodifications may be made without departing from the spirit and scope ofthe present inventions. Thus, the present inventions are intended tocover alternatives, modifications, and equivalents, which may beincluded within the spirit and scope of the present inventions asdefined by the claims.

What is claimed is:
 1. A method for treating a patient using amulti-channel neurostimulation system, the method comprising: deliveringa plurality of pulsed electrical waveforms respectively within aplurality of timing channels of the neurostimulation system, therebytreating the patient; predicting sets of stimulation pulses within theelectrical waveforms that will potentially overlap temporally; andsubstituting each of the potentially overlapping pulse sets with areplacement stimulation pulse, such that each replacement stimulationpulse is delivered within at least one of the respective timingchannels, thereby preventing temporal overlap between the stimulationpulses of the respective electrical waveforms while preventing frequencylocking between the timing channels.
 2. The method of claim 1, whereinsubstituting each potentially overlapping pulse set with the replacementstimulation pulse comprises selecting one of the stimulation pulses ofeach potentially overlapping pulse set as the replacement stimulationpulse.
 3. The method of claim 2, further comprising determining therelative amplitude or pulse duration of the stimulation pulses withineach potentially overlapping pulse set, wherein the one stimulationpulse of each potentially overlapping pulse set is selected as therespective replacement stimulation pulse based on the respectivedetermined relative amplitude or pulse duration.
 4. The method of claim2, wherein the selection of the one stimulation pulse of eachpotentially overlapping pulse set alternates between the respectivetiming channels.
 5. The method of claim 2, further comprising assigningone of the timing channels as a high priority timing channel, whereinthe one stimulation pulse of each potentially overlapping pulse setassociated with the high priority timing channel is selected as thereplacement stimulation pulse.
 6. The method of claim 5, wherein thetiming channels are respectively associated with different tissueregions, and the timing channel associated with the tissue region thatwould be more adversely affected by dropping a stimulation pulse withinthe timing channel is assigned as the high priority timing channel. 7.The method of claim 2, wherein one stimulation pulse of at least one ofthe potentially overlapping pulse sets is a cathodic pulse, and anotherstimulation pulse of the at least one potentially overlapping pulse setis an anodic pulse, and the cathodic pulse is selected as thereplacement stimulation pulse.
 8. The method of claim 1, furthercomprising defining each of the replacement stimulation pulses as afunction of the stimulation pulses within the respective potentiallyoverlapping pulse set.
 9. The method of claim 8, wherein the functioncomprises an average of the amplitudes and/or durations of thestimulation pulses within the respective potentially overlapping pulseset.
 10. The method of claim 8, wherein the function comprises asummation of the amplitudes of the stimulation pulses within therespective potentially overlapping pulse set.
 11. The method of claim 1,wherein the stimulation pulses of each of the potentially overlappingpulse sets have the same polarity.
 12. The method of claim 1, whereineach replacement stimulation pulse is delivered within all of therespective timing channels.
 13. The method of claim 1, wherein thepulsed electrical waveforms are delivered via a common electrode. 14.The method of claim 1, wherein the pulsed electrical waveforms havedifferent pulse frequencies.
 15. The method of claim 1, furthercomprising defining the pulsed electrical waveforms in response to auser input.
 16. The method of claim 1, further comprising: predicting acharge recovery pulse and a stimulation pulse within the electricalwaveforms that will potentially overlap temporally; and dropping ortemporally shifting at least a portion of the charge recovery pulse,thereby preventing temporal overlap between the charge recovery pulseand the stimulation pulse of the respective electrical waveforms.
 17. Amulti-channel neurostimulation system, comprising: a plurality ofelectrical terminals configured for being respectively coupled to aplurality of electrodes; analog output circuitry configured fordelivering a plurality of pulsed electrical waveforms respectivelywithin a plurality of timing channels to the electrical terminals; andcontrol circuitry configured for predicting sets of stimulation pulseswithin the pulsed electrical waveforms that will potentially overlaptemporally, and substituting each of the potentially overlapping pulsesets with a replacement stimulation pulse, such that each replacementstimulation pulse is delivered within at least one of the respectivetiming channels, thereby preventing temporal overlap between thestimulation pulses of the respective pulsed electrical waveforms. 18.The neurostimulation system of claim 17, wherein the control circuitryis configured for substituting each potentially overlapping pulse setwith the replacement stimulation pulse comprises by selecting one of thestimulation pulses of each potentially overlapping pulse set as thereplacement stimulation pulse.
 19. The neurostimulation system of claim18, wherein the control circuitry is further configured for determiningthe relative amplitude or pulse duration of the stimulation pulseswithin each potentially overlapping pulse set, wherein the controlcircuitry is configured for selecting the one stimulation pulse of eachpotentially overlapping pulse set as the respective replacementstimulation pulse based on the respective determined relative amplitudeor pulse duration.
 20. The neurostimulation system of claim 18, whereinthe one stimulation pulse of each potentially overlapping pulse setalternates between the respective timing channels.
 21. Theneurostimulation system of claim 18, wherein the control circuitry isfurther configured for assigning one of the timing channels as a highpriority timing channel, and selecting the one stimulation pulse of eachpotentially overlapping pulse set associated with the high prioritytiming channel as the replacement stimulation pulse.
 22. Theneurostimulation system of claim 21, wherein the timing channels arerespectively associated with different tissue regions, and wherein thecontrol circuitry is configured for assigning the timing channelassociated with the tissue region that would be more adversely affectedby dropping a stimulation pulse within the timing channel as the highpriority timing channel.
 23. The neurostimulation system of claim 18,wherein one of the stimulation pulses of at least one of the potentiallyoverlapping pulse sets is a cathodic pulse, and another pulse of the atleast one potentially overlapping pulse set is an anodic pulse, and thecontrol circuitry is configured for selecting the cathodic pulse as thereplacement stimulation pulse.
 24. The neurostimulation system of claim17, wherein the control circuitry is further configured for definingeach of the replacement stimulation pulses as a function of thestimulation pulses within the respective potentially overlapping pulseset.
 25. The neurostimulation system of claim 24, wherein the functioncomprises an average of the amplitudes and/or durations of thestimulation pulses within the respective potentially overlapping pulseset.
 26. The neurostimulation system of claim 24, wherein the functioncomprises a summation of the amplitudes of the pulsed within therespective potentially overlapping pulse set.
 27. The neurostimulationsystem of claim 17, wherein the stimulation pulses of each of thepotentially overlapping pulse sets have the same polarity.
 28. Theneurostimulation system of claim 17, wherein the analog output circuitryis configured for delivering each replacement stimulation pulse withinall of the respective timing channels.
 29. The neurostimulation systemof claim 17, wherein the analog output circuitry is configured fordelivering the pulsed electrical waveforms via a common electrode. 30.The neurostimulation system of claim 17, wherein the pulsed electricalwaveforms have different pulse frequencies.
 31. The neurostimulationsystem of claim 17, wherein the control circuitry is further configuredfor defining the pulsed electrical waveforms in response to a userinput.
 32. The neurostimulation system of claim 17, wherein the controlcircuitry is further configured for predicting a charge recovery pulseand a stimulation pulse within the electrical waveforms that willpotentially overlap temporally, and dropping or temporally shifting atleast a portion of the charge recovery pulse, thereby preventingtemporal overlap between the charge recovery pulse and the stimulationpulse of the respective electrical waveforms.
 33. The neurostimulationsystem of claim 17, further comprising the plurality of electrodes. 34.The neurostimulation system of claim 17, further comprising a housingcontaining the plurality of electrical terminals, analog outputcircuitry, and control circuitry.